Adsorption from the liquid phase has long been used for removal of contaminants present at low concentrations in process streams. The commercial use of adsorption for the recovery of major components of feed streams as pure products, sometimes termed bulk separations, is a comparatively recent development.
Examples of bulk separation include the separation of linear paraffins from branched-chain cyclic hydrocarbons, separation of olefins from paraffins, and the separation of C8 aromatic isomers, which include xylenes and ethylbenzene. Typically these processes use zeolitic adsorbents because of the particularly useful selectivities developed, but the technology and theory are equally applicable to suitably selective nonsieve adsorbents such as alumina, charcoal, metal oxides, and so on.
Development of large-scale bulk separations from the liquid phase has been accomplished by the use of a flow scheme simulating the continuous countercurrent flow of absorbent and process liquid, without actual movement of the adsorbent (“simulated moving bed” or SMB). See, for instance, U.S. Pat. No. 2,985,589, and Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition (1978), Vol. 1, page 563-581). Such processes have been developed for the separation of p-xylene from a mixture of C8 aromatics (UOP Parex™), n-paraffin separation (UOP Molex™), olefin-paraffin separation (UOP Olex™). Variants have been developed, such as the Toray Aromax™ process for p-xylene separation. Each these products have important and well-known uses, e.g., p-xylene for the production of polyester fibers and plastics.
SMB have recently also been used for separation on a smaller scale, such as for separating pharmaceuticals, biochemicals, and fragrances. By way of example, see WO 2003026772.
In essentially all of these adsorptive separation units using a simulated countercurrent movement of the adsorbent and the feedstream, the simulation involves holding the adsorbent in place in one or more cylindrical adsorbent chambers. The positions at which the streams involved in the process enter and leave the chambers are slowly shifted along the length of the beds by means of, for example, a rotary valve, which functions on the same principle as a multi-port stopcock. Normally there are at least four streams (feed, desorbent, extract and raffinate) employed in this procedure and the location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shifted in the same direction at set intervals. Each shift in location of these transfer points delivers or removes liquid from a different bed within the chamber. For p-xylene separations, the extract stream is composed of p-xylene and desorbent and the raffinate stream is composed of desorbent and the p-xylene-depleted xylene mixture.
This shifting could be performed using a dedicated line for each stream at the entrance to each bed. However, this would greatly increase the cost of the process and therefore the lines are reused and each line carries one of the four process streams at some point in the cycle. In SMB, the feedstream is connected to a series of beds in sequence, first to bed no. 1, then to bed no. 2, and so forth for numerous beds, generally being between 12 and 24. These beds may be considered to be portions of a single large bed whose movement is simulated. Each time the feedstream destination is changed, it is also necessary to change the destinations (or origins) of at least three other steams, which may be streams entering the beds, such as the feedstream, or leaving the beds, such as the extract and raffinate. Desorbent and various flushes may also enter and leave the beds. The moving bed simulation may be simply described as dividing the bed into series of fixed beds and moving the points of introducing and withdrawing liquid steams past the series of fixed beds instead of moving the beds past the introduction and withdrawal points. See U.S. Patent Application 2008036913.
The general technique employed in the performance of SMB technology is well described in the literature. For instance a general description directed to the recovery of p-xylene was presented at page 70 of the September 1970 edition of Chemical Engineering Progress (Vol. 66, No 9). A generalized description of the process with an emphasis on mathematical modeling was given at the International Conference on “Fundamentals of Adsorption”, Schloss Elmau, Upper Bavaria, Germany on May 6-11, 1983 by D. B. Broughton and S. A. Gembicki. U.S. Pat. No. 4,029,717 issued to F. J. Healy et al., describes a SMB process for the recovery of p-xylene from a mixture of xylene isomers. Numerous other available references describe many of the mechanical parts of an SMB system, including rotary valves for distributing various liquid flows, the internals of the adsorbent chambers and control systems, e.g., see Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition (1978), Vol. 1, previously cited, especially p. 569 et. seq.; and FIG. 4 therein; FIG. 2.6.4 in Meyers' Handbook of Petroleum Refining Processes (3rd Edition (2004), McGraw-Hill Handbooks, pg. 2.51).
With regard to the rotary valve, U.S. Pat. No. 2,985,589 describes the idea of moving ports on fixed beds and an accompanying rotary valve to distribute stream flows among the fixed beds. See also U.S. Pat. Nos. 3,040,777; 3,192,954; 3,422,848. Processes utilizing a rotary valve in an SMB process are described in numerous such as U.S. Pat. Nos. 3,201,491 and 3,291,726. In U.S. Pat. No. 3,706,812, columns are linked together through tees connected to rotary valves, as shown in FIG. 1 of the patent. The disclosed system incorporated a check valve between each column and its tee to maintain correct directional flow. The patent also disclosed the use of solenoid valves to move the ports through the columns. A rotary valve as used in SMB may be described as accomplishing the simultaneous interconnection of at two separate groups of conduits.
The cyclical advancement of the streams through the solids may also be accomplished by utilizing a manifold arrangement to cause the fluid to flow in a counter current manner with respect to the solids. The valves in the manifold may be operated in a sequential manner to effect the shifting of the steams in the same direction as overall fluid flow throughout the adsorbent solids. See U.S. Pat. No. 3,706,812.
UOP Sorbex™ Processes, which include the Parex™, Molex™, and Olex™ processes described above, makes use of a rotary valve that typically distributes net in streams (feed and desorbent), net out streams (extract and raffinate), and assorted flushes (primary flush in, secondary flush in and flush out) to and from the appropriate sieve beds inside the sieve chambers in the SMB unit. The net in, net out and assorted flush streams are sequentially cycled through the various bed lines. The assorted flush streams are necessary to avoid contamination caused by the sharing of the bed lines from the other net in and net out streams. Sorbex™ Process units typically process several streams that have significantly different compositions. While the terms extract and raffinate are relative terms depending on the nature of the components being separated, the preference of the solids, and the nature of the apparatus or system, as used herein the term “extract” will mean a stream comprising product and desorbent and the term “raffinate” will mean a stream comprising by-products and desorbent.
The use of SMB is important for the separation of xylenes and especially p-xylene from a mixture of xylenes and ethylbenzene. See U.S. Pat. No. 3,686,342 and U.S. Pat. No. 3,510,423. Parex™ Process units, which make use of the rotary valve in the aforementioned SMB process, typically can be fed mixtures of xylenes of various concentrations, e.g., equilibrium xylenes, a concentrated p-xylene stream from a selective toluene disproportionation unit, filtrate from a crystallizer which is low in p-xylene concentration, and mixtures thereof. In the conventional configuration, all of the feedstreams are mixed together and sent to a rotary valve as one feed. As taught in U.S. Pat. No. 5,750,820, feedstreams may be kept separate and fed to different beds based their composition.
Numerous methods have been devised to increase the efficiency and/or productivity of the rotary valve in combination with adsorptive separation processes, such as taught in U.S. Pat. Nos. 4,434,051; 5,470,464; 5,750,820; 5,912,395; 7,208,651; U.S. Patent Applications 20060848065 and 20080149565.
However, with the necessity of increasing sources of feed streams used in a refinery and/or chemical plant, there is still a need for a system that can process a variety of feed sources at the same time without major increases in expensive new apparatus and/or without loss of efficiency. By way of example, there is the need to integrate new feedstreams having differing compositions, for instance feedstreams having higher concentrations of p-xylenes than equilibrium provided by a mixture of xylenes, such as provided by the Mobil Selective Toluene Disproportionation (STDP™) or Mobil Toluene to P-xylene (MTPX™) processes, providing feedstreams having >90 wt % p-xylene (see U.S. Pat. Nos. 4,274,982; 4,851,604; 5,365,003; 5,498,822), with xylene feedstreams having the equilibrium concentration of about 23 wt % para-xylene, and/or feedstreams having very low amount (about 3-6 wt %) of para-xylene, such as provided by crystallizer filtrate. Mixing the feedstreams is inefficient since at least one of the feedstreams is caused to have a decreased concentration of the desired extract. In addition, in current practice at least some of the bedlines and or beds are not in use at any given time, which is inefficient use of a very expensive resource.
The present inventors have surprisingly discovered that by providing parallel rotary valves configured or plumbed to operate independently provide, in embodiments, increased capability for additional feed and flush streams, which in embodiments substantially results in either increasing the capacity of a unit or decrease the energy requirement of a unit at constant capacity. This allows the designer and/or operator to optimize multiple feed locations, maintain or increase the number of flushes, such as to flush raffinate from the bed lines and flush desorbent into the sieve chambers between the raffinate and desorbent.